What action should be taken when the battery voltage is too low for external power application?

When the battery voltage is too low for external power application, the most appropriate action is to notify maintenance personnel. This step is crucial because maintenance personnel have the expertise and tools necessary to diagnose and troubleshoot the underlying issues related to the battery. Simply attempting to recharge the battery without proper assessment could lead to further complications or damage, especially if the battery is not in a condition to accept a charge safely.

Waiting for a few minutes may not resolve the issue, as the battery's voltage may not recover on its own. Using a backup power source could also be an inappropriate workaround without first understanding the condition of the primary battery. Hence, notifying maintenance ensures that the situation is handled appropriately and safely, maintaining the operational integrity of the aircraft.